1. Field of the Invention
The present invention relates to a fuel-injection control device for a diesel engine, particularly to a fuel-injection control device for a diesel engine using a common-rail-type fuel-injection pump, and in more detail to an optimum control device for the combination between a fuel-injection control and EGR (Exhaust Gas Recirculation) control in a diesel engine using a common-rail-type fuel-injection pump.
2. Description of the Related Art
A common-rail-type fuel-injection pump (also referred hereinafter to "a common-rail-type pump") has recently been developed, which is constructed to inject pressurized fuel from a reservoir, i.e., a so-called common-rail and allows the pressure of the injected fuel to be arbitrarily set and controlled in relation to engine operating conditions such as engine speed, engine load, etc. (see, for example, Japanese Unexamined Patent Publications No. 62-75025 and No. 2-191865). A variety of advantageous methods have been contrived for applying such a common-rail-type fuel-injection pump to a diesel engine. For example, there have been proposed the methods for its utilization such as: the pressure of injected fuel is controlled according to an operating state of the engine and the timing and the interval of fuel-injection are also controlled according to an operating state of the engine (see, for example, Japanese Unexamined Patent Publication No. 62-645); the pressure of injected fuel is controlled on the basis of an engine load (see, for example, Japanese Unexamined Patent Publications No. 62-75025 above-mentioned and No. 63-50649); and so on.
On the other hand, in the purification of exhaust gas of a diesel engine, the engine cannot use a three-way reducing and oxidizing catalyst converter as in a gasoline engine, due to its excess air ratio, but it uses an EGR control for reducing NOx emissions. Practical techniques have yet been elucidated for minimizing the emissions of exhaust gases particularly in the case of combining the fuel-injection control using such a common-rail-type pump with the EGR control. For example, even though the EGR control is simply added to the technique of increasing the pressure of injected fuel as the engine load is increased as described in the above-mentioned Japanese Unexamined Patent Publication No. 62-75025, it will be impossible to produce the minimum amount of NOx, black smoke, PM (Particulate Matter), etc. emissions.
Furthermore, specially in a diesel engine using the EGR control, it is necessary to stop the EGR control when the engine is cold in order to prevent the corrosion of cylinder bores and the like during such an interval. However, there is encountered the problem of a remarkably increased amount of NOx emission if the EGR is off when the engine is cold. With respect to a jerk pump which is widely used in a diesel engine at present and individually mounted for each cylinder of the engine, the only method for suppressing the increase of NOx when the engine is cold is to provide a delay in the fuel-injection timing. The delay in the fuel-injection timing when the engine is cold is, however, not practicable since it brings about an unstable ignition conjointly with a cold combustion chamber, resulting in such disadvantages as the vibration of the engine, the emissions of white smoke and an offensive smell, and so on.
Accordingly, the above-mentioned, proposed control method for a common-rail-type fuel-injection pump is intended to perform an optimum fuel control according to the operational state of the engine so as to prevent the output power of the engine from decreasing and to improve the fuel consumption ratio, but they take no consideration of reducing the emissions of NOx, smoke, etc. Effective methods for utilization of the common-rail-type fuel-injection pump are therefore hardly elucidated with the view of taking advantage of this common-rail-type pump to provide optimum control of a diesel engine combined with purification of the exhaust gas. As described above, particularly in the case of combining a fuel-injection control using this common-rail-type pump with an EGR control, it is an actual fact that any practical technique is not able to minimize the emissions of exhaust gases.